1. Field of the Invention
The present invention relates to a disc array apparatus and in particular, to a disc array apparatus capable of coping with a difference in access performance of disc (or disk) units (DKU) constituting a disc array of the disc array apparatus.
2. Description of the Related Art
A disc array apparatus has a performance characteristic greatly affected by performance characteristics of physical disc units (DKU) involved in the disc array. However, it is preferable that a disc array apparatus have a constant performance characteristic not affected by the physical disc units. Conventionally, a disc array apparatus has used disc units having similar performance characteristics.
These years, however, a SCSI (small computer system interface) disc apparatus has begun to be used. The SCSI disc apparatus has been rapidly improving its performance characteristics. Such a physical disc apparatus has a short life cycle on the market and can be used only for a short period of time. That is, for the disc array apparatus, a countermeasure should be taken to cope with a difference in the performance among the disc apparatus units involved in the array.
In a conventional disc array apparatus, when a disc unit is to be replaced with a new one, it is difficult to find a disc apparatus having the performance presupposed by the disc array apparatus. Accordingly, it is not easy to add a physical disc unit in an array.
That is, it is difficult to prepare a plurality of physical disc units having an identical performance. When a disc unit is to be added or modified, the disc array apparatus has a problem in maintaining a constant data access with respect to a host apparatus.
It is therefore an object of the present invention to provide a disc array apparatus capable of maintaining a constant performance characteristic as a disc array apparatus even if a disc unit having a different performance is added or if a disc unit is modified to have a different performance.
The disc array apparatus according to the present invention comprises a plurality of disc units for storing a data transmitted from a host apparatus, and an array controller for controlling upon an access from the host apparatus a data transfer between the disc units and the host apparatus, the array controller including: a performance detector for detecting performance values of the plurality of disc units; a correction value generator for calculating a correction value based on a difference between the performance value detected for each of the disc units and a predetermined reference performance value; and a response timing corrector for correcting a timing at which a data transfer end report is to be sent to the host apparatus using the correction value obtained by the correction value generator.
In the disc array apparatus, the performance detector determines a performance characteristic for each of the disc units involved. The performance characteristic includes a response speed for an access, for example. Furthermore, the correction value generator compares a value actually measured to a reference value as a value presupposed by the array controller to obtain a difference as a correction value. In general, after a disc array apparatus is implemented for use, a higher-speed disc unit is developed. When such a higher-speed disc unit is mounted on the disc array apparatus, the correction value generator generates a correction value comparing the reference value of the disc array apparatus to an actually obtained performance value. Next, the response timing corrector, when an access is made from the host apparatus, delays a data transfer end report to the host apparatus until a timing based on the correction value. That is, for the host apparatus, a data transfer appears to be carried out with a response time presupposed by the disc array apparatus.